With the rapid development in science and technology, more and more portable electronic devices such as mobile phones, digital cameras, tablet computers, and notebook computers or the like have become essential tools in people's lives. Since portable electronic devices are generally used by individuals and have certain privacy, data stored inside the electronic devices (such as phone books, photos, personal information) is privately owned. Once the electronic device is lost, the data may be used by others, causing unnecessary losses. Although password protection has been used to prevent the electronic device from being used by others, passwords may be easily leaked or cracked, thus having a low degree of security. Moreover, the user has to remember the passwords so that the electronic device can be used. If the passwords are forgotten, a lot of inconveniences are brought to the user. Therefore, a personal fingerprint identification system has been developed for the purpose of identity verification so as to improve data security.
A pixel array circuit is used in a fingerprint identification system to sense contact of a finger, and the capacitance between the pixel array circuit and the finger is converted by the pixel array circuit into a pixel output signal. Since the pixel output signal is an analog signal, the pixel output signal needs to be converted into a digital signal before being transmitted to a fingerprint determination module at the back end to determine whether the pixel array circuit corresponds to a finger ridge or a finger valley of the fingerprint. In the fingerprint identification system, the power consumption of the pixel array circuit is large when a fingerprint is captured. Generally, the pixel array circuit is usually in a sleep mode, and it will enters the normal operation mode to perform fingerprint identification only when it is touched by user's finger, thereby achieving the effect of power saving.
Therefore, how to determine whether the pixel array circuit is touched by user's finger has become one of goals to be achieved in the industry.